Donna attends an auction of unclaimed freight and an unknown woman asks Donna to bid on a Chinese horse. Donna wins the auction but the woman disappears. Donna brings the item home, but then strange things begin to happen.
Trisha is now part of the family and fits in easily. Jeff enjoys playing with her, and Donna likes having another child to care for. Mary realizes she is the one stuck with babysitting Trisha instead of going on dates.
Alex and Jeff wake up early to go camping with their friend Red. As Donna sees them out the door, Red comments that women cannot go camping because they are too soft. Donna tells Mary they are going camping with the guys to prove Red wrong.
Both Mary and Jeff are looking forward to the upcoming dance. Mary has plans with Scotty, and Jeff has plans with Becky. But when they get to the dance, it seems Scotty and Becky only have eyes for each other.
Donna and Jeff accompany Alex to Chicago where he has a medical conference. They plan to attend the baseball game on the weekend because Don Drysdale is playing. Jeff is unable to get in touch with Don and is disappointed.
Trisha's friend Nancy is being spoiled by her grandfather with too many presents so her parents forbid her getting anymore. But the grandfather decides to involve the Stones by sending the gifts for Nancy to their house.
The Stones are excited when Mary returns home after a long absence but she seems to only have time for her friends. After one social engagement too many Jeff explains to his how disappointed their parents are.
Jeff and his friends start a club using their walkie talkies to communicate. One day, another voice intrudes on their conversations, a girl who won't give her name. This drives him to find out who she might be, anxiously looking around at school, but no luck. Eventually, Jeff and Smitty use a radio wave locator to find her.